The field of this invention is that of providing fluid power to operate subsea components such as the shear rams of subsea blowout preventers and similar components. These components typically make up what is called a subsea blowout preventer stack and have a high volume requirement to operate an appropriate number of these functions. It can range up to 200 gallons of accumulated capacity necessary to operate various blowout preventers and valves on a subsea blowout preventer stack. In many cases such as with shear rams the pressure required to stroke the shear rams to the point of contacting the pipe to be sheared is relatively low (i.e. 500 p.s.i.) and then the force required to shear the pipe is relatively high (i.e. 5000 p.s.i.).
This is further complicated by the fact that an accumulator typically pressurizes the fluid by having compressed gas such as nitrogen provide pressure on the fluid. The compressibility of the gas allows a substantial volume of fluid to be pressurized and then discharged under pressure. A disadvantage of this is that as the liquid is discharged from the accumulator, the volume of the gas becomes larger and therefore the pressure of the gas and liquid becomes lower. As the pistons and rams of the blowout preventer move forward and need higher pressure to do their functions, the pressure of the powering fluid becomes lower. This has typically meant that the lowest pressure from the accumulator must exceed the highest operational pressure of the system. The highest pressure of the accumulator to make this work is simply higher. When a higher pressure is provided by the accumulator than is needed, it is simply throttled to reduce the pressure and turn the energy into heat.
This has been the nature of the operations of subsea accumulators for the past 50 years. There has been a long felt need for more accumulator volume capacity and the only way that those skilled in the art have met the challenge is with larger and higher pressure accumulators.